With the rapid popularization of wireless communication devices, the demand for a high frequency filter with small size, low mass and good performance is growing, and a piezoelectric acoustic filter made on silicon wafer has been widely accepted by the market. Piezoelectric acoustic resonators constituting such high-performance filter mainly include Film Bulk Acoustic Resonators (FBAR) and Solid Mounted Resonators (SMR).
The resonant frequency of a piezoelectric acoustic resonator is decided by thickness of each layer in a propagation path and the sound velocity of a longitudinal sound wave in each layer. The resonant frequency is mainly affected by thickness of a piezoelectric layer and the sound velocity of the piezoelectric layer. Thicknesses of two electrodes and the sound velocity within the two electrodes also affect the resonant frequency greatly. Effect of an acoustic reflector constituted by the cavity on the resonant frequency can be ignored, for the acoustic reflector can reflect almost all of acoustic energies back to the piezoelectric layer. If the acoustic reflector is constituted by alternate arrangement of a high acoustic impedance layer and a low acoustic impedance layer, the top layer of the reflector will contain a small part of acoustic energies, so that the function of the reflector will be contributed to the resonant frequency to some extent.
Thicknesses of the piezoelectric layer, metal or dielectric layer of the piezoelectric acoustic resonator and the sound velocity within the piezoelectric layer, metal or dielectric layer are all changed with the temperature, thus the resonant frequency of the piezoelectric acoustic resonator is also changed with the temperature. Though thickness expansion or shrinkage generated from the change with the temperature in each layer will affect the resonant frequency, the change of an acoustic wave propagation velocity with the temperature within each layer is the main factor to affect the resonant frequency of the piezoelectric acoustic resonator to be changed with the temperature. At present, most of materials applied in the piezoelectric acoustic resonator present a negative temperature coefficient of sound velocity, that is, the sound velocity will decrease with the rise of temperature, for the materials will be “softened” at a higher temperature (e.g., the across-atomic force is weakened). The decrease of the across-atomic force will cause the decrease of a material elastic coefficient, thereby lowering the sound velocity. For example, a temperature coefficient of sound velocity of Aluminum Nitride (AlN) is −25 ppm/° C., and a temperature coefficient of sound velocity of Molybdenum (Mo) is −60 ppm/° C.
Generally a Radio Frequency (RF) filter constituted by the piezoelectric acoustic resonator has a passband frequency response, the Temperature Coefficient of Frequency (TCF) of the piezoelectric acoustic resonator will reduce the manufacturing yield rate of the RF filter, for devices or components constituted by the piezoelectric acoustic resonator can only meet requirements of the passband bandwidths within a certain temperature range. In most of the required applications of duplexer, in order to still meet the requirement within a very wide temperature range, a low temperature coefficient of frequency is very important. A high stable oscillator containing the piezoelectric acoustic resonator has more strict requirements on the temperature coefficient of frequency of the piezoelectric acoustic resonator, and the temperature coefficient of frequency is required to be extremely low or approximate to zero, for most of oscillators are used to provide reference signals or timing signals, and it demands that the change of temperature exerts tiny influence on these signals.
In order to obtain a low temperature coefficient of frequency, a common method is to add one layer of silicon dioxide (SiO2), materials of temperature compensation layer, in a stacked structure of the piezoelectric acoustic resonator. Major materials forming the stacked structure of the piezoelectric acoustic resonator all have a negative temperature coefficient of sound velocity, but the SiO2 materials have a positive temperature coefficient of sound velocity, and by adjusting thicknesses of the SiO2 and materials of all other layers in the stacked structure, the drift of the frequency of the piezoelectric acoustic resonator with the temperature can be effectively reduced in certain filter applications. However, it is required that the resonant frequency of the piezoelectric resonator, a electromechanical coupling coefficient, a resonator quality factor and the temperature coefficient of frequency of the resonator meet certain requirements simultaneously in some other filter applications, which is hard to be achieved by simply adjusting the thickness of each layer in the stacked structure of the resonator. Therefore, how to implement a temperature compensation capability that can adjust the materials of the temperature compensation layer at present, that is, adjusting the temperature coefficient of sound velocity of the materials of the temperature compensation layer, so that the process of designing a resonator have a stronger flexibility, has become a technical problem required to be solved urgently at present.